The term "integrated optics" refers in general to a class of devices for guiding and controlling light in thin film layers or in narrow waveguiding channels formed in suitable dielectric materials (crystal or glass). The present use of integrated optic devices is confined primarily to laboratory systems and development prototypes. However, numerous applications in communications and in optical sensing are anticipated. Representative IO devices include optical waveguides, switches, polarizers, phase modulators, and other functional devices. In system applications several functional units may be combined ("integrated") on a single crystalline or glass substrate (an "optical chip"), and the devices may be connected to sources, detectors or other optical devices by means of optical fiber.
While several materials have been used as substrates for IO device fabrication, one of the most widely used IO device materials is lithium niobate. It is used primarily because of its favorable optical and electro-optical properties, and it is anticipated that IO devices of lithium niobate will find wide application. However, to make the transition from the laboratory to usage in practical applications, a number of packaging problems will require solution.
One such problem arises because lithium niobate exhibits strongly anisotropic thermal expansion properties: that is to say the dimensional changes in the material associated with a temperature change differ in different directions in the crystal. In practical application, devices may be required to survive wide temperature fluctuation, e.g. from -55.degree. C. to +150.degree. C. is a common military specification. A mismatch in thermal expansion between the IO chip and the substrate on which it is mounted can result in substantial stresses being transmitted to the IO device. In severe cases, these stresses can lead to failure of the bond between the IO device and the mounting or to breakage of the brittle crystalline chip.
Lithium niobate also exhibits a photoelastic effect whereby an applied stress results in a change in the optical refractive index. Many of the desired functions of integrated optic devices depend on controlled changes in the refractive index. For example, controlled phase modulation in lithium niobate can be achieved by application of an electric field (the electro-optic effect). Stresses due to thermal expansion mismatch resulting from uncontrolled temperature excursions can, via the photoelastic effect, lead to spurious refractive index changes which can interfere with the controlled changes being impressed in the desired operation of the device. For these and other reasons it is desirable to provide a mounting for the IO device whereby stress transmission is minimized. As will be shown below, provision for such low stress mounting is further complicated by the anisotropic thermal expansion previously mentioned.
Lithium niobate has a crystal structure which exhibits a three-fold rotational symmetry about an axis in a particular direction in the crystal. By convention a Cartesian coordnate system of three mutually orthogonal axes designated X, Y and Z is used to describe the crystal's physical tensor properties with the Z axis directed along the crystal's three-fold rotation axes.
It is sufficient to note that thermal expansion coefficients range from 2.times.10.sup.-6 /.degree.C. to 7.5.times.10.sup.-6 /.degree.C., while the thermal expansion in the X-Y plane is isotropic and in the range of 14-17.times.10.sup.-6 /.degree.C. An 8-1 difference. Another useful material for IO devices is lithium tantalate (LiTaO.sub.3). This material has a thermal expansion coefficient of about 4.times.10.sup.-6 /.degree.C. in the Z direction and 16.times.10.sup.-6 /.degree.C. in the X and Y directions).
If an IO device is fabricated in a thin rectangular slab of LiNbO.sub.3 such that X and Z or Y and Z axes are in the plane of the slab, then the thermal expansion coefficient differs strongly along the two principal axes in the plane of the slab. While a conventional metal substrate may match one coefficient (e.g., certain brass or stainless steel compositions match the high thermal expansion coefficient of LiNbO.sub.3) a severe mismatch would exist between the metal substrate and the lithium niobate in the direction of the low thermal expansion coefficient axis. The problem does not exist in chips cut such that the X and Y axes lie in the principal plane. However, the electro-optic effect is anisotropic also, and for optimum optical function it is frequently desirable to have anisotropic thermal expansion in the plane of the slab. However, optical properties of LiNbO.sub.3 such as the electro-optic effect are anisotropic also, and for reason of optimum optical function it is frequently desirable to utilize a crystal cut such that anisotropic thermal expansion exists in the plane of the slab.
Typically thin slabs are cut from large crystal boules such that one of the principal axis is normal to the broad face of the slab, and the slabs are designated X-, Y-, or Z-cut accordingly as the X, Y or Z axis is orthogonal to the broad face. Furthermore, rectangular slabs are frequently cut such that principal axes lie along the edges of the slab. In such case the slab may be further specified by noting the axis along the long direction. Thus and XY-cut describes a crystal with X axis normal to the broad face and Y axis in the long direction of the slab.